Hybrid electric vehicles ("HEVs") utilize both an internal combustion engine and one or more electric machines (e.g., motors/generators) to generate power and torque. The electric motor/generator within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering the driver-demanded torque and is also typically used to start the vehicle's engine.
One type of hybrid electric vehicle utilizes an integrated starter/alternator or starter/generator which functions as both a motor and a generator. The starter/generator typically provides a variety of functions and benefits, including but not limited to starting the vehicle's engine; generating and providing electrical power to vehicle systems and components; providing additional torque to the vehicle's engine during heavy accelerations; assisting in and/or performing the braking functions of the vehicle, thereby capturing the kinetic energy of the moving vehicle; and stopping or "shutting off" the vehicle's engine during idling and decelerating states or conditions, thereby improving the vehicle's overall fuel economy.
In order to start the vehicle's engine, the starter/generator receives power from an onboard electrical power supply such as a battery. In one type of hybrid electric vehicle, a rechargeable forty-two volt (42 V) battery, such as a lead acid or nickel-metal-hydride battery, is used to supply electrical power to the starter/generator. While these types of batteries are often effective to provide the requisite electrical charge to power the starter/generator, they suffer from some drawbacks.
For example and without limitation, during cold temperature starting conditions, these batteries provide only limited performance. The power pulse capability for a typical forty-two volt battery at -20 degrees Celsius is often less than fifty percent of its power pulse capability at 20 degrees Celsius. Therefore, if a typical forty-two volt battery is sized to meet cost and weight constraints, it may have marginal cold temperature pulse discharge performance and may be unable to meet engine cranking loads.
Efforts have been made to overcome the cold temperature starting limitations associated with these types of batteries. These efforts include utilizing different battery technologies in parallel, self-heating the battery core by use of resistive elements, and providing an auxiliary climate control system for the battery. These efforts, however, have not produced satisfactory results. For example, the utilization of different battery technologies requires additional costly and complex electronics to address battery management issues. Furthermore, the use of resistive elements and auxiliary climate control systems results in an excessive amount of battery self-discharge which adversely impacts the fuel economy gains that are provided by the starter/generator.
There is therefore a need for a method of preconditioning a battery to improve cold temperature starting of a hybrid electric vehicle which overcomes the drawbacks of prior methods, strategies and systems.